Tetrafluorammonium tetrafluoroborate (NF.sub.4 BF.sub.4) is used as a common ingredient in formulations for oxidizer source solid gas generators which find particular utility in combustion driven chemical lasers.
It has been demonstrated that tetrafluorammonium tetrafluoroborate may be prepared by metathesis of NF.sub.4 SbF.sub.6 with CsF or AgF in anhydrous hydrogen fluoride to yield CsSbF.sub.6 or AgSbF.sub.6 as a precipitate and tetrafluorammonium bifluoride which is in turn reacted with boron fluoride to yield tetrafluorammonium tetrafluoroborate. The net reactions for cesium fluoride, for instance, may be written as EQU NF.sub.4 SbF.sub.6 +CsF .sup.HF CsSbF.sub.6 .dwnarw.+NF.sub.4 HF.sub.2
removal of the solid precipitate and treatment of the liquid phase which BF.sub.3 forms the BF.sub.4 salt by the reaction EQU NF.sub.4 HF.sub.2 +BF.sub.3 .sup.HF NF.sub.4 BF.sub.4 +HF
excess hydrogen fluoride is removed by evaporation.
Subject to strict safety precautions, NF.sub.4 AsF.sub.6 may also be reacted with RbF to yield RbAsF.sub.6 as the precipitate as a predicate to forming NF.sub.4 BF.sub.4 from NF.sub.4 HF.sub.2.
The main deficiency in the procedure is the cost of the added metal fluoride. The present cost of cesium fluoride is about $15,100 per pound mole and the cost of silver fluoride $15,500 per pound mole.
A need exists to find a lower cost metal fluoride which enables the conduct of the metathesis reaction at an equal or greater degree of completion.